1. Field of the Invention
The present invention relates to a method for detecting the point of origin of a position sensor used, for example, for focusing a lens of an optical system, for positioning a work table of a machine tool, or the like.
2. Description of the Related Art
It is generally known that when a table of a machine tool or the like having a workpiece positioned thereon is displaced by a certain distance in an X-axis or Y-axis direction, the distance of such displacement is commonly measured by the use of a position sensor, such as a linear scale, so that the position of the workpiece on the table may be determined. Such position sensors may comprise a movable unit anchored to the table (the table in this example being the object member to be measured) and a stationary unit anchored to a base of the position sensor. As the table is displaced, the movable unit is also displaced with respect to the stationary unit. The length of the displacement of the table is calculated on the basis of the relative movement between the moveable and stationary units.
In accomplishing the above measurement, the reference position for the position sensor used for setting the measuring position to an absolute value, i.e., for detecting the point of origin, is conventionally determined as follows. A light shield plate is fixedly attached to the movable unit, and an end point sensor, such as a photo interrupter, is positioned in proximity with the position sensor. As the shield plate is displaced with the movable unit towards the end point of displacement of the moveable unit, the shield plate ultimately intercepts light to the photo interrupter to consequently change the output of the end point sensor, which, for example, may thereby be switched from an off to an on condition. It has been customary heretofore that the change point of the output of the end point sensor is regarded as the reference point or point of origin of the position sensor.
Thus, in order to set the position of the position sensor to an absolute value using conventional methods, it was necessary to use an external end point sensor, such as a photo interrupter, in order to determine the reference position for the absolute value. Consequently, the position detecting system as a whole is rendered bulky. Such units also involve other problems, including a relatively high cost of production.
Furthermore, when using an end point sensor, some variations in the output of the end point sensor may be experienced due to temperature fluctuations, environmental changes and so forth. Such variations may result in variations of position determinations made based on the above mentioned reference point. Consequently, the precision of this positioning point suffers in terms of accuracy.
In systems where position detection is performed by quantizing the output of a position sensor, in principle, a quantization error of plus or minus one pulse may exist. In other words, the absolute position is only accurate to a variation equivalent to the quantization error. of course, the precision of an absolute value so determined may-be adversely affected by the quantization error.